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555 monostable circuit

Discussion in 'General Electronics Discussion' started by kelvinmead, Nov 22, 2011.

  1. kelvinmead

    kelvinmead

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    Nov 22, 2011
    hi,

    delving into the world of 555 (at least until my pic(s) arrive) i wish to create a variable timer, or to be more accurate, a timer that can be finely tuned.

    i wish to dispense a perfect amount of liquid through a fast acting solenoid.

    the liquid is not viscous, or to be more accurate is not more viscous than water, but i need to dispense between 22-25ml each time

    the setup in the basic 555 monostable timer has a resistor and a capacitor controlling the length of time that the solenoid will stay open for, but in the world of best practice / usual standard would people use a relevant variable resistor, or a relevant variable capacitor?

    thanks
    kelvin.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,191
    2,693
    Jan 21, 2010
    Use a variable resistor. In the sizes that you require, variable capacitors either do not exist, or would be huge and expensive.
     
  3. kelvinmead

    kelvinmead

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    Nov 22, 2011
    yeah, im flicking across rs now, and they variable caps they do are real small value differences!

    thanks!
     
  4. davenn

    davenn Moderator

    13,290
    1,756
    Sep 5, 2009
    hi Kelvin
    Welcome :)

    just note what steve said .... keep the capacitor value fixed for any given time range and use a variable resistor.
    In fact you are not likely to find large value variable caps, practically they dont exist.
    all the variable caps you will find will be down in the few pF to 100's of pF area. I dont recall ever seeing a variable cap over ~ 1000pF and you need one with a value a lot larger than that.

    cheers
    Dave
     
    Last edited: Nov 22, 2011
  5. kelvinmead

    kelvinmead

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    Nov 22, 2011
    was quite easy in the end, and im probably lucky to have an rs components to hand (albeit usually making do with parts that they have in stock)

    went with a 47MuF and 100k variable giving me the 0.5 - 5 minutes delay i needed!

    thanks!
     
  6. lapistola

    lapistola

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    Nov 22, 2011
    Can you keep a solenoid energized for 5mins without it burning out??

    I have played with a few and burnt nearly all out by expecting them to stay energized for a long period of time.
     
  7. davelectronic

    davelectronic

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    Dec 13, 2010
    Hi there. I would have thought keeping a solenoid energized would depend on what its operating mechanically, and power levels, if its with in the units working voltage, and the current of the device is kept with in its safe working limits, i would think it could stay activated for some time, but not sure about indefinitely with out some kind of duty cycle.
     
  8. kelvinmead

    kelvinmead

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    Nov 22, 2011
    the solenoid is usually closed, power to make it open, and im only opening it for 0.5 - 2 seconds at a time, but according to the datasheet the ones i have are designed to run at 12v constant (lifetime infinity it says) 24v 1 minute, 50v 12 seconds.

    now, i have an addition to this project.

    when i hit the activation switch, as well as it setting off the solenoid, it lights a selection of leds (8-10) in order one at a time.

    ive looked at the 4017 decade counter after following http://www.allaboutcircuits.com/vol_6/chpt_7/6.html but it only seems to work when you manually hit the trigger (i was hoping for a sequence per hit)

    i could build a 555 monostable into a 555 astable and then into the 4017 to get the desired output, but will be ugly! is there a secret smaller way of triggering... maybe the use of a crystal and slowing it down?

    anyway, enough rambling!

    kelvin.
     
  9. davelectronic

    davelectronic

    1,079
    12
    Dec 13, 2010
    Hi again kelvinmead.
    I cant take you to the next step, or rather i would be unsure of what route to take, i am sorry about that, regarding the solenoid if the mechanical load is not exceeded it should perform with in the data sheet spec, hopefully another member can continue with ideas around the led sequence and solenoid synchronization. :)
     
  10. lapistola

    lapistola

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    Nov 22, 2011
    Could the action of the solenoid hit a micro switch when it activates? Or use a DPDT switch that will trigger both circuits at the same time.
     
  11. davelectronic

    davelectronic

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    Dec 13, 2010
    Yes i dont see any problem with micro switches DPDT and or even to control relays, but how about the time on to off needed, or is this part of the circuit taken care of already ? :)
     
  12. lapistola

    lapistola

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    Nov 22, 2011
    The monostable is triggered so will only need a momentary type switch. After looking at the 4017 circuit couldn't you switch the solenoid with a relay on the 555 output and also have the 555 output send its power to the V+ rail of the 4017 circuit?
     
  13. davelectronic

    davelectronic

    1,079
    12
    Dec 13, 2010
    I would think its depending on the relays current demands, the second bit of sending the voltage power to the V+ rail of the 4017 i am not sure on, of if it throws any issues up with either IC. :)
    PS, ive looked at the 4017 and cant see any issues, but not totally sure.
     
  14. lapistola

    lapistola

    37
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    Nov 22, 2011
    Good point!

    The 4017 does have a few components and with the current a relay coil may need it could burn the 555. How about the 555 output switches a transistor which then closes the relay which 12v output then splits, one to the solenoid and one to the 4017 but through a voltage splitter (2 resistors) to give it the 6v it needs (or a voltage regulator if the 4017 will work on 5v). This then eliminates over current risk through the first 555.
     
  15. davelectronic

    davelectronic

    1,079
    12
    Dec 13, 2010
    It sounds reasonable your component count goes up, but if thats not a problem, i cant see why that wont work, careful choice of transistor. :)
     
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